Proximity detector and alarm utilizing field effect transistors

ABSTRACT

A proximity detector and alarm in which an antenna is connected to the gate of a metal oxide semiconductor field effect transistor (MOSFET) which causes a silicon controlled switch (SCS) to trigger a blocking oscillator.

Waited States Patent 1191 Healey et a1.

[54] PROXIMITY DETECTOR AND ALARM UTILIZING FIELD EFFECT TRANSISTORS Inventors: Gerald F. Healey, Seaside Heights; Joseph C. Nirschl, West Long Beach, both of NJ.

The United States of America as represented by the Secretary of the Army, Washington, D.C.

Filed: Aug. 29, 1969 Appl. No.: 854,084

Assignee:

US. (:1 ..340/258 1), 307/251, 340/38, 340/384 E m. (31.; ..G08b 13/26 Field of Search ..340/258 0, 228.1, 340/258 D; 307/251 111 3,733,597 1451 May 15, 1973 [56] References Cited UNITED STATES PATENTS 3,255,380 6/1966 Atkins et al ..340/258 C X 3,301,307 1/1967 Nishigaki et al. ..340/228.1 X 3,508,238 4/1970 Baker ..340/258 C Primary ExaminerDavid L. Trafton Attorney Harry M. Saragoyitz, Edward J. Kelly, Herbert Berl and James I. Busch [57] ABSTRACT A proximity detector and alarm in which an antenna is connected to the gate of a metal oxide semiconductor field effect transistor (MOSFET) which causes a silicon controlled switch (SCS) to trigger a blocking oscillator.

7 Claims, 3 Drawing Figures Patented May 15, 1973 INVENTORS GERALD F HEALEY JOSEPH C. NIRSCHL BY I #04 5m 9- ,n MM M 7AM T Hwrh [/Vgenf) ATTORNEYS,

Patented May 15, 1973 3,733,597

2 Sheets-Sheet 2 (V) l/ j x INVENTORS GERALD Fr HEALEY BY 741 JOSEPH C- NIPSCHL I PROXIMITY DETECTOR AND ALARM UTILIZING FIELD EFFECT TRANSISTORS BACKGROUND OF INVENTION The present invention relates to an alarm system and in particular to an alarm using a MOSFET as a basic element thereof. Proximity detectors in the past have had problems insofar as they were too complex or required a large amount of power consumption when used for a long period of time. This was because an active sensor was used which required a constant power dissipation.

SUMMARY The general purpose of the invention is to provide a proximity detector which embraces all the advantages of similarly employed devices and possesses none of the disadvantages. To-this end an antenna is connected to the gate of a MOSFET. Physical motion of an object in proximity to the antenna causes a transient effect on the gate, presumably due to a change in the effective gate capacitance. By utilizing a MOSFET a simple, small size, low cost, low power consumption circuit can be designed which will have a completely passive sensor.

DESCRIPTION OF THE DRAWING The exact nature of the invention will be readily apparent from consideration of the following specification relating to the annexed drawings in which:

FIG. 1 is a schematic of the basic detector circuits;

FIG. 2 is a schematic of the detector circuit with a particular alarm circuit; and

FIG. 3 is a schematic of the detector circuit with cable driver for recording of target signatures.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a MOSFET 11 the gate of which is connected to antenna 12. A forward bias is established by the gatedrain connection via a high megohm resistor 13 which may be of the order of ohms. The physical motion of an object in proximity to the antenna causes a transient effect on the gate, due to a change in the effective gate capacitance 14 thereby inducing a transient excursion of the drain current from the normal quiescent operating point.

FIG. 2 illustrates a circuit which, rather than trigger an alarm, indicates the transient signal produced by a vehicle passing on a road. A coaxial cable 19 is used to transmit the signal from a roadside antenna to a recorder 20. A filter 21 is used between the cable 19 and recorder to suppress 60 H2 pick-up noise. Zener diodes 23 and 24 serve as voltage regulators.

FIG. 3 uses the basic FIG. 1 circuit combined with an audible alarm circuit. O2 is a silicon controlled switch (SCS) which becomes triggered by positive excursions of the FET drain voltage capacitively coupled through 16 to its gate. The anode load of the SCS is an alarm circuit consisting of a blocking oscillator with transistor Q3 and miniature speaker 17 producing a chirping sound. The alarm section and reset switch 18 are separated from the antenna section 'by ft. cable connection 19. This is to provide for reset since otherwise the operators proximity would trigger the circuit. Sensitivity will vary with atmospheric humidity, rate of ap proach and the sole type of an individual wearing shoes. A circuit was made that was sensitive to a walking person 6 feet away.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that numerous modifications or alterations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

For instance in FIG. 3 two antenna boxes in parallel or push-pull arrangement could be used with a differential amplifier in the recorder. In this way, the signal to noise ratio may be increased. The antenna boxes would be placed at a certain separation distance from each other on the side of the road.

Also in road traffic monitoring the MOSFET detector could be combined with a solid-state subcarrier oscillator and transmitter to telemeter the data to a central monitoring station to be recorded there.

We claim:

1. A proximity detector comprising:

a metal oxide semiconductor field effect transistor (MOSFET) having gate, drain, and source electrodes;

an antenna connected to said gate electrode,

a high megohm resistor connected between said gate and said drain electrode,

a voltage source,

a load resistor connected between the negative end of said voltage source and the point common to the high megohm resistor and the drain electrode;

a coaxial cable,

a transistor having a base, emitter and collector,

said collector connected to the negative terminal of said voltage source,

said base connected to said drain electrode and,

said emitter connected to a first end of said cable;

and

a recording means connected to a second end of said cable.

2. The circuit of claim 1 further comprising a filter connected between said second end of said cable and the recording means.

3. The circuit of claim 2 in which said filter is used to suppress 6O HZ.

4. The circuit of claim 1 further comprising:

a zener diode connected between said source electrode and the positive terminal of said voltage source and,

a zener diode connected between the first end of said cable and the positive terminal of said voltage source.

5. An alarm circuit comprising:

a MOSFET having gate, drain and source electrodes;

an antenna connected to said gate electrode,

a high megohm resistor connected between said gate and said drain electrode,

a cable at least ten feet in length,

a silicon controlled switch having a first electrode connected to said drain electrode, a second electrode connected to said source electrode, a third electrode connected to ground and a fourth electrode connected to one end of said cable;

a blocking oscillator connected between the second end of said cable and said source electrode and,

a reset switch connected between said blocking oscillator and the second end of said cable.

6. The circuit of claim 5 in which the blocking oscillator output is a loudspeaker.

7. The circuit of claim 5 in which a variable resistor is connected between said drain electrode and the first electrode of said silicon controlled switch. 

1. A proximity detector comprising: a metal oxide semiconductor field effect transistor (MOSFET) having gate, drain, and source electrodes; an antenna connected to said gate electrode, a high megohm resistor connected between said gate and said drain electrode, a voltage source, a load resistor connected between the negative end of said voltage source and the point common to the high megohm resistor and the drain electrode; a coaxial cable, a transistor having a base, emitter and collector, said collector connected to the negative terminal of said voltage source, said base connected to said drain electrode and, said emitter connected to a first end of said cable; and a recording means connected to a second end of said cable.
 2. The circuit of claim 1 further comprising a filter connected between said second end of said cable and the recording means.
 3. The circuit of claim 2 in which said filter is used to suppress 60 HZ.
 4. The circuit of claim 1 further comprising: a zener diode connected between said source electrode and the positive terminal of said voltage source and, a zener diode connected between the first end of said cable and the positive terminal of said voltage source.
 5. An alarm circuit comprising: a MOSFET having gate, drain and source electrodes; an antenna connected to said gate electrode, a high megohm resistor connected between said gate and said drain electrode, a cable at least ten feet in length, a silicon controlled switch having a first electrode connected to said drain electrode, a second electrode connected to said source electrode, a third electrode connected to ground and a fourth electrode connected to one end of said cable; a blocking oscillator connected between the second end of said cable and said source electrode and, a reset switch connected between said blocking oscillator and the second end of said cable.
 6. The circuit of claim 5 in which the blocking oscillator output is a loudspeaker.
 7. The circuit of claim 5 in which a variable resistor is connected between said drain electrode and the first electrode of said silicon controlled switch. 